The present invention relates to apparatus for producing a flux of charge carriers.
Apparatus for producing a flux of charge carriers have a variety of uses in many different applications. They may be put to use in displays for consumer electronics, in imaging and diagnostic systems used in research and development applications, in switching and amplifying circuits in sensors and in lithography systems for manufacturing.
In the field of imaging and lithography, apparatus for producing a flux of charge carriers may be used as proximity probes as well as to generate electron beams.
One type of proximal probe is a scanning tunnelling microscope (STM).
A STM comprises a tip that is scanned over the surface of a specimen. Electrons are emitted from the tip and tunnel into the specimen. The rate of tunnelling is highly sensitive to the separation of the tip and specimen and the measured current, together with the position of the tip, may be used to build an image of the specimen.
Although STM imaging has very high resolution, it has several disadvantages. For example, the tip is mechanically, rather than electrically, scanned across the specimen, thus reducing its raster speed compared with electron beam systems. Furthermore, image information may only be obtained from primary electrons, in this case, electrons tunnelling between the tip and the specimen. This may be compared with other electron imaging systems in which image information may also be obtained from secondary electrons that are generated when sufficiently energetic primary electrons strike the specimen. Another disadvantage is that the specimen must be arranged a few nanometers from the tip of the STM for electron tunnelling to occur. This is difficult to achieve and attempts to position the tip often result in the tip crashing into the specimen.
An example of using an STM in lithography is given in xe2x80x9cHybrid atomic force/scanning tunnelling lithographyxe2x80x9d by K. Wilder, H. T. Soh, A. Atalar and C. F. Quate, Journal of Vacuum Science and Technology, volume B15, pp 1811-1817 (1997).
Another type of apparatus for producing a flux of charge carriers and which may be used for imaging and lithography is microcolumn electron beam system. A microcolumn is a miniaturised version of a conventional electron beam system and an example of a microcolumn is given in xe2x80x9cExperimental evaluation of a 20xc3x9720 mm footprint microcolumnxe2x80x9d by E. Kratschmer, H. S. Kim, M. G. R. Thomson, K. Y. Lee, S. A. Rishton, M. L. Yu, S. Zolgharnain, B. W. Hussey and T. H. P. Chang, Journal of Vacuum Science and Technology, volume B14, pp 3792-3796 (1996).
A microcolumn comprises a field emitter, a beam-forming source lens, scanning electrodes and a beam focussing objective lens. The electrode and lenses are arranged around the axis of the column only a few millimeters long. The microcolumn operates with beam voltages of the order of 100-1000 volts.
One advantage of a microcolumn over larger electron beam systems is that lens aberration is reduced. Furthermore, an array of microcolumns separated from one another by a few millimeters may be used in parallel to expose the surface of a wafer. However, microcolumns have several disadvantages. For example, a microcolumn operates under high vacuum conditions, requiring the sample to be kept in a vacuum. This prohibits imaging of a sample in air, which would be advantageous in biological applications. Furthermore, microcolumns are generally complex and expensive to manufacture. In addition, the minimum beam voltage that can be used is limited by space charge, where electrons within the beam repel each other and cause the beam to broaden. A broader beam results in loss of resolution for lithography and imaging.
Electron beams can also be used to determine the composition of sample. For example, conventional scanning electron microscopes (SEMs) may be used to perform energy dispersive X-ray (EDX) spectroscopy and wave dispersive spectroscopy (WDS). However, the instruments used for these types of spectroscopy are cumbersome and require keeping in a vacuum. This prevents analysis of large samples or specimens that can only be analysed in air.
Apparatus for producing a flux of charge carriers may be used in flat panel displays. One type of device is a field emitting display device used in flat panel displays. An example of such a device may be found in U.S. Pat. No. 5,955,850 and comprises an array of field emitters, each comprising a conical cathode and an extractor electrode, and a common anode. The device may be fabricated using standard microelectronic processing techniques on a common substrate. However, the device has several disadvantages. The space between the cathode and the anode must be evacuated and the operative voltage is in the region of 1000""s of volts.
Recently, a field emission device has been developed that has a much reduced operating voltage. The device is described in xe2x80x9cNanoscale field emission structures for ultra-low operation at atmospheric pressurexe2x80x9d by A. A. G. Driskill-Smith, D. G. Hasko and H. Ahmed, Applied Physics Letters, volume 71, pp 3159-3161 (1997). The device comprises an emitter having a tip radius of about one nanometer and a closely configured extractor electrode. This laboratory experimental device allows field emission to occur at very low voltages and the emitted electrons to travel ballistically from the emitter tip to the extractor electrode even under atmospheric conditions.
This device has been modified to produce a nanoscale electron tube and is described in xe2x80x9cThe xe2x80x9cnanotriodexe2x80x9d: A nanoscale field-emission tubexe2x80x9d by A. A. G. Driskill-Smith, D. G. Hasko and H. Ahmed, Applied Physics Letters, volume 75, pp 2845-2847 (1999). The device comprises a nanometer-scale chamber comprising an emitter and a closely configured gate electrode, sealed under vacuum by an integrated anode. One of the advantages of the triode is that it has a low operating voltage, while the gate electrode may be used to control the anode current.
However, such a triode is not suitable for generating an electron beam outside of the device because the structure is sealed.
The present invention seeks to provide an improved apparatus for producing a flux of charge carriers.
According to the present invention there is provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter having a nanometer scale tip radius on a common substrate with an extractor arranged no more than 50 nm from the emitter to extract charge carriers therefrom and a specimen adjacent the source, to receive a flux of charge carriers from the source.
The emitter may have a tip radius less than 2 nm or less than 1 nm.
According to the present invention there is also provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter and an extractor to extract charge carriers from the emitter, wherein the emitter and the extractor are configured on a common substrate and a specimen, wherein the emitter and the specimen are arranged in a near-field configuration.
In the near-field configuration phase coherence of the charge carriers may be substantially maintained.
The near-field configuration may comprise an arrangement whereby the emitter and the specimen are disposed less than 200 nm from each other.
The extractor may be arranged no more than 50 nm from the emitter.
The extractor may be arranged no more than 30 nm from the emitter.
According to the present invention there is still further provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter and an extractor to extract charge carriers from the emitter, wherein the emitter and the extractor are configured so as to allow extraction of charge carriers under a gaseous atmosphere without ionisation of the gas and wherein a specimen adjacent the source, to receive a flux of charge carriers from the source.
The emitter and extractor may be configured such that said charge carriers are extracted while a bias is applied to the extractor relative to the emitter.
The relative applied bias may be positive and between 7 to 20 V.
According to the present invention there is further provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter and an extractor to extract charge carriers from the emitter and configured to extract charge carriers while a turn-on bias of less than 100V is applied to the extractor relative to the emitter and a specimen adjacent the source, to receive a flux of charge carriers from the source.
The turn-on bias may be less than 10V.
The apparatus may include a bias source to apply a bias to the specimen relative to the emitter.
The bias applied by the source to the specimen may be positive relative to the emitter and between 14 to 40 V relative to the emitter.
The emitter may comprise a metal, for example tungsten.
The emitter may comprise a tip member, for example comprising an alloy of gold and palladium, with a radius less than 2 nm.
The extractor may comprise tungsten and may comprise a sheet having an aperture.
The diameter of the aperture may be less than 100 nm or 50 nm.
The source may further comprise means for collecting charge carriers.
The source may further comprise means for deflecting flux of charge carriers.
The source may further comprise means for focussing the flux of charge carriers.
The flux of charge carriers may be a charge carrier beam.
The apparatus may be configured to operate in air at atmospheric pressure.
The charge carriers may be electrons.
The emitter and the specimen may be disposed less than 200 nm from each other.
According to the present invention there is also provided a method of producing a flux of charge carriers, the method comprising providing a source comprising configuring an emitter having a nanometer scale tip radius on a substrate with an extractor arranged no more than 50 nm from the emitter to extract charge carriers therefrom and wherein providing a specimen adjacent the source, to receive a flux of charge carriers from the source.
According to the present invention there is also provided a method of fabricating a source, the method comprising depositing a thin film and allowing said thin film to coalesce into individual particles.